FWIW, I did order a number of pulleys from KB a while back and have measured them for reference. Although the 9# pulley is on the car (in storage), perhaps you can get something out of these numbers:
6# - 3-1/4"
7# - 3-1/8"
8# - 3"
9# - ? (on car 30 miles away) but likely in the area of 2-7/8" given above numbers
14# - 2-3/8"
You say you went from 3-3/4"? Very strange because the biggest pulley in my set, the 6# part, is only 3-1/4" which is the size to which you say you went. 3-3/4" is huge...that'd be like a 4# pulley. If you went from a 4# pulley to a 6# pulley you might not see much difference.
Where'd that large pulley come from and where did you source the 3-1/4" pulley you were led to believe is a 9# unit?
When I bought the blower, I thought it had the 9 on it. When it was getting dynoed at Evolution Performance they realized it was gonna make 14psi and go boom. They had a 3 3/4" there and used that to tune it and it made 6psi.
So then I called Kenne Bell and asked them for the 9PSI and they said they think its the 3 1/4. But I do have the 2 3/8 so that pulley is at least the right one.